Sensor for Measurement of Fuel Rod Gas Pressure during Loss-of-Fluid-Tests

Qualification tests have been conducted of a measurement system for determining the pressure of certain fuel rods in the loss-of-fluid-test (LOFT) reactor. Because of physical size (0.35-in. OD by 5.5-in. length) and operational characteristics, an eddy current device was selected as the most promising measurement transducer for the application. The sensor must operate at pressure up to 17.2 MPa (2500 psig) and at temperatures up to 800°F. During the reactor transient caused by loss of coolant flow, sensor temperature and applied pressure will vary rapidly and significantly. Consequently, qualification tests included subjection of the sensor to rapid depressurization, temperature transients, and blowdowns in an autoclave, as well as to calibrations and various slow temperature cycles. Calibration data show the sensor responds linearly for applied pressure, but with both temperature-dependent sensitivity and zero pressure output signal. Temperature compensation of signal output values was achieved with use of a derived temperature-dependent algorithm to transform the dc output voltage to indicate pressure. Because of the very rapid (up to 100°F/s) temperature changes imposed, the sensor erroneously indicated pressure by a maximum of 18% of reading during some transient tests. In addition to the influence of magnitude and rate of temperature change on the observed pressure measurement error, a very significant factor pertains to the spatial distribution of the temperature field immediately adjacent to the sensor envelope. As proven during 700 hours of qualification tests, precision and stability of the sensor should be satisfactory for LOFT.